Underground-structure construction method

ABSTRACT

An underground-structure construction method of arranging an underground structure by causing drilling apparatuses engaged with a concrete-made closed-shaped bottom frame to travel in a reciprocating manner to drill a just-underground of a bottom part of the underground structure, discharging to an earth surface excavated earth and sand of an internal ground, and extending a building frame of an upper part of the underground structure includes: open-cutting a ground to form a larger-sized mortar-shaped working hole; forming a closed-shaped horizontal reference area in an edge ground; arranging a bottom frame on the horizontal reference area; building a first stage building frame on the bottom frame; and backfilling, with backfill soil, an annular-shaped outer space of inner and outer spaces as a result of dividing the working hole, the drilling apparatuses being caused to uniformly drill, from the horizontal reference area as a drilling initiation plane, the just-underground of the bottom part.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority upon Japanese Patent Application No. 2016-055400 filed on Mar. 18, 2016, of which full contents are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a method of constructing an underground structure such as an underground multistory bicycle parking system, an underground multistory parking system, or a harmful substance burial facility. In particular, the present invention relates to a method of causing a plurality of drilling apparatuses to drill a just-underground of a bottom part of the underground structure so that the underground structure is allowed to sink and is resultantly arranged underground.

Description of the Background Art

The applicant previously proposed a self-propelled drilling apparatus for such sort of purpose (Patent Documents 1, 2). This drilling apparatus includes a travelable travelling body and a rotary cutter arranged in the travelling body. A plurality of drilling apparatuses are arranged at a bottom frame of an underground structure so as to be reciprocatable at intervals. The plurality of drilling apparatuses are configured such that they can simultaneously travel in a horizontal direction along the bottom frame to thereby discharge drilled earth inwardly on an internal side of the underground structure while drilling a just-underground of a bottom part of the underground structure. The rotary cutter can perform an over-excavation drilling beyond a lateral width of the bottom frame or a thickness of a building frame thickness of the underground structure.

Description will be made with reference to FIG. 6. It has been known that, when an underground structure 10 is arranged underground, a plurality of subsidence control apparatuses 20 arranged on a ground are used to maintain the horizontality of the underground structure 10 (Patent Documents 3, 4). This subsidence control apparatus 20 includes: a rope 21 having a tip end connected to the building frame of the underground structure 10; a winch 22 configured to wind the rope 21 around a feed drum; a post 23 for anchoring the rope 21 through a plurality of pulleys; a tension detection means for detecting tension of the rope 21; and a control section configured to control the winch 22 based upon a detection result of the tension detection means. The tip ends of their respective ropes 21 extending from the plurality of winches 22 are connected to the building frame of the underground structure 10. The control section controls feed amounts of their respective ropes 21 of the winches 22 such that detection results of the tension detection means with respect to the ropes 20 are uniform in tension.

Conventionally, the underground structure 10 were arranged underground through the following steps.

[1] Assembly of Drilling Facility

(a) Installation step of a subsidence control apparatus

A foundation pile 24 is constructed at a planned installation position of the subsidence control apparatus 20. A concrete foundation 25 is constructed at an earth-surface side of the foundation pile 24. On the concrete foundation 25, the winch 22 is provided and the post 23 is erected.

(b) Installation Step of a Bottom Frame

In parallel with setting the subsidence control apparatus 20, a bottom frame 11 of steel material such as H steel is disposed on the earth surface in accordance with a planar shape of the underground structure 10. Both ends of their respective steel materials are joined to form the bottom frame 11 to have a closed shape.

(c) Assembly Step of a Drilling Apparatus

The bottom frame 11 is assembled with the plurality of drilling apparatuses 30 at intervals thereamong. A part of the ground is drilled in advance so that a rotary cutter 31 of each of the plurality of drilling apparatuses 30 can be inserted into below the bottom frame 11.

(d) Construction Step of an Underground Structure

Bars are arranged on the bottom frame 11 and formworks are assembled to be opposed to one another. Concrete is disposed in the formwork to construct the underground structure 10 having a height of approximately 2 to 3 m. During this, the rope 21 drawn from the plurality of subsidence control apparatuses 20 is buried in the underground structure 10 and connected.

[2] Repetition of Drilling/Excavation Work and Expansion Work of Underground Structure

The plurality of drilling apparatuses 30 are caused to travel in a reciprocating manner so as to drill the just-underground of the underground structure 10. The removal of such drilled earth and sand discharged to an internal side of the underground structure 10 together with excavated earth and sand of the internal ground on the internal side of the underground structure 10 is performed by earth removal means such as a clam shell. Such drilling and excavation work and removal work are performed repetitively.

The drilling apparatus 30 performs a drilling operation to such an extent that the drilled length is beyond the width of the building frame of the underground structure 10 so that a non-contact relation is generated between an outer wall surface of the underground structure 10 and a hole wall thereby allowing the underground structure 10 to sink due to a self-weight.

In parallel with the sinking of the underground structure 10 or after the sinking of the underground structure 10, a building frame for extension is additionally arranged on an upper part of the underground structure 10.

[3] Step of Automatically Controlling Drilling and Sinking

When the existing structure 10 is allowed to sink, the control of the feed amounts of the ropes 21 at the respective winches 22 based upon a result of the tension detection of the ropes 21 and the automatic control of e.g. the rotation direction, the rotation rate, the traveling speed, the rotation direction of the drilling apparatus 30 are performed in parallel so that the ropes 21 connected to a plurality of positions of the underground structure 10 can have uniform tension, thereby maintaining the horizontality of the underground structure 10.

[4] Step of Filling Fine Grain Aggregate

As required, a space formed between the width-expansion drilled hole wall and the outer wall surface of the underground structure is filled with a fine grain aggregate to thereby reduce frictional resistance between the wall faces.

PRIOR ART DOCUMENTS Patent Documents

-   Patent Document 1: Japanese Patent Laid-open Publication No.     2011-017161 -   Patent Document 2: Japanese Patent Laid-open Publication No.     2011-052484 -   Patent Document 3: Japanese Patent Laid-open Publication No.     2006-144387 -   Patent Document 4: Japanese Patent Laid-open Publication No.     2010-150811

Problems to be Solved

The conventional technique to construct an underground structure had points to be improved as shown below.

(1) A plurality of high-cost subsidence control apparatuses 20 had to be provided. In order to support each of such a plurality of subsidence control apparatuses 20, there was a further need to construct the foundation pile 24 and the concrete foundation 25, which resulted in a higher construction cost.

(2) If the horizontality of the underground structure 10 was lost in the middle of the sinking, the correction of the horizontality was very difficult. In order to maintain the horizontality of the underground structure 10, the sinking velocity of the underground structure 10 had to be maintained at low, which left room for improvement of the construction.

(3) In order to install the subsidence control apparatus 20 on the ground, a wide installation space was required at the exterior of the underground structure 10. For this reason, the subsidence control apparatus 20 could not be installed in a field site not having such an installation space.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above points. It is an objective of the present invention to provide an underground-construction method of constructing an underground structure that can be efficiently arranged underground with a low cost while maintaining the underground structure having a high horizontality with a simple methodology.

Means for Solving Problems

The present invention provides an underground-structure construction method of arranging underground an underground structure by: causing a plurality of drilling apparatuses, which are reciprocatably engaged with a bottom frame of closed shape made of concrete of the underground structure, to travel in a reciprocating manner so as to drill a just-underground of a bottom part of the underground structure; discharging, to an earth surface, excavated earth and sand of an internal ground on an internal side of the underground structure; and extending a building frame of an upper part of the underground structure, the underground-structure construction method comprising the steps of: open-cutting a ground so as to form a mortar-shaped working hole larger in size than the underground structure; forming a horizontal reference area of closed shape in an edge ground of a bottom part of the working hole; arranging a bottom frame on the horizontal reference area in the working hole so as to form the bottom frame into a closed shape; building a first stage building frame of the underground structure on the bottom frame in the working hole; and backfilling, with backfill soil, an annular-shaped outer space out of inner and outer spaces as a result of dividing the working hole at the underground structure, wherein the plurality of drilling apparatuses are caused to uniformly drill, from the horizontal reference area as a drilling initiation plane, the just-underground of the bottom part of the underground structure.

As further aspect of the present invention, the bottom frame has a continuous lower flange formed thereon, and a self-weight of the underground structure is supported, through an entire lower surface of the lower flange, by the just-underground of the bottom part of the underground structure.

As still further aspect of the present invention, in parallel with causing the plurality of drilling apparatuses engaged with the bottom frame to uniformly drill the just-underground of the bottom part of the underground structure, the internal ground on the internal side of the underground structure is excavated so that excavated earth of the internal ground is discharged to the earth surface.

As still further aspect of the present invention, the plurality of drilling apparatuses engaged with the bottom frame are caused to perform a width-expansion drilling beyond a width of the building frame of the underground structure, so that the underground structure is allowed to sink while a space is formed for insulation between a hole wall as a result of the width-expansion drilling and an outer wall surface of the underground structure.

As still further of the present invention, each of the plurality of drilling apparatuses includes: a self-propellable travelling body engaged with the bottom frame; and a rotary cutter arranged in the travelling body rotatably around a vertical axis, so that each of the plurality of drilling apparatuses drills the just-underground of the bottom part of the underground structure through the rotary cutter's rotation and travel along the bottom frame.

Advantageous Effects Achieved by the Invention

The present invention provides any one of the following advantageous effects.

(1) A simple method is used to merely arrange underground a horizontal reference area formed with a high horizontality in advance at the bottom part of the working hole as a reference area on which the underground structure is to be disposed. This method can efficiently allow the horizontal reference area to sink with a low cost while maintaining the high horizontality of the underground structure during the sinking.

(2) According to the present invention, the underground structure can be allowed to sink while maintaining the high horizontality. This consequently eliminates any of a high-cost subsidence control apparatus used in the conventional construction method and a concrete foundation or a foundation pile for supporting this. A shorter construction period and a significantly-reduced construction cost can therefore be achieved.

(3) Since the width-expansion drilling beyond the thickness of the building frame of the underground structure is possible, a space for insulation can be formed between the hole wall subjected to the expansion drilling and the outer wall surface of the underground structure. This can also eliminate an operation carried out in the conventional construction method to fill the space with a fine grain aggregate in order to reduce the frictional resistance.

(4) Not only during the initial sinking of the underground structure but also during the sinking thereof, the self-weight of the underground structure can be uniformly supported by the just-underground of the bottom part of the underground structure through an entire surface of the bottom rail. The high horizontality of the underground structure can therefore be maintained.

(5) Since the working hole is formed to have a larger shape than the outer shape of the underground structure, the space of the working hole can be used to smoothly and efficiently perform the construction operations of the underground structure of the first step such as an operation to assemble a reinforcing bar, an operation to assemble a formwork, and an operation to disassemble the formwork.

BRIEF DESCRIPTION OF THE DRAWINGS

For more thorough understanding of the present invention and advantages thereof, the following descriptions should be read in conjunction with the accompanying drawings in which:

FIG. 1 depicts a longitudinally cross-sectional view showing an underground structure.

FIG. 2 depicts a longitudinally cross-sectional view showing the bottom part of the underground structure.

FIG. 3 depicts an explanatory view of a method with respect to: an open-cutting operation of a working hole shown in (a); an installation operation of the bottom frame and a construction operation of the underground structure shown in (b); and a backfilling operation of an annular space shown in (c).

FIG. 4 depicts an expanded view showing the installation operation of the bottom frame.

FIG. 5 depicts a cross-sectional view showing the bottom part of the underground structure obtained by placing concrete in the formwork.

FIG. 6 depicts a conventional underground-structure construction method.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The following sections will describe the present invention in detail with reference to the drawings.

[1] Underground Structure

Description will be made with reference to FIGS. 1, 2. The underground structure 10 is a concrete-made box having a horizontal cross section having e.g. a rectangular shape, a polygonal shape, a circular shape, or an elliptical shape. The building frame thickness and its entire size are appropriately selected depending upon the application of the underground structure 10.

[1.1] Bottom Frame

The lower end of the underground structure 10 has the bottom frame 11. The underground structure 10 is supported to be abutted to the ground through the bottom frame 11.

In this example, a case will be described in which the bottom frame 11 is constituted by H-like steel obtained by connecting a pair of upper and lower flanges 12 and 13 by a web 14. However, another configuration also may be used in which the pair of upper and lower flanges 12 and 13 has therebetween connected two webs 14 with an interval thereamong.

The bottom frame 11 may have a continuous lower flange 13. Any contact surface structure may be used so long as the structure allows the underground structure 10's self-weight to be supported by the just-underground of the bottom part of the underground structure 10 through the entire lower face of the lower flange 13.

[1.2] Incidental Equipment of Bottom Frame

Description will be described with reference to FIGS. 2 and 4. The inner faces of the inner sides of the upper and lower flanges 12 and 13 have a pair of guide rails 15 opposed to each other to guide the drilling apparatus 30. At the center of the inner faces of the inner sides of the web 14, a reaction force receiver 16 such as a straight gear is continuously provided in order to obtain the self-propelled reaction force of the drilling apparatus 30.

[2] Drilling Apparatus

The drilling apparatus 30 for drilling the just-underground of the underground structure 10 may be a known apparatus.

The following section will describe the drilling apparatus 30 shown in FIG. 2. The drilling apparatus 30 is a self-propelled apparatus that has both of a function to scrape and drill the just-underground at the lower end of the underground structure 10 and a function to discharge the drilled soil to the inner side of the underground structure 10.

The drilling apparatus 30 includes a travelable travelling body 35 engaged with the guide rails 15 of the lower end 21 of the underground structure 10, a travel means for the travelling body 35, a rotary cutter 31 provided in the travelling body 35, and the rotation means for the rotary cutter 31.

[2.1] Travelling Body

The travelling body 35 is a horizontal stand that is anchored so as to be travelable to the bottom frame 11 while including therein the rotary cutter 31.

The travelling body 35 has a plurality of idling rollers 36 and can be engaged with the guide rails 15 through the idling rollers 36.

The travelling body 35 includes a driving source 32 such as a motor and a gear-like travel wheel 33.

The travel wheel 33 is engaged with the reaction force receiver 16 of the bottom frame 11. The driving source 32 provided in the travelling body 35 drives the travel wheel 33 to invert the forward and backward directions of the travel wheel 33, thereby self-propelling the drilling apparatus 30 along the bottom frame 11.

[2.2] Rotary Cutter

The rotary cutter 31 includes at least a rotation axis 31 a penetrating the travelling body 35 in a longitudinal direction for axial support and a plate-like horizontal rotor 31 b provided in the rotation axis. The lower face and tip end of the horizontal rotor 31 b have a plurality of drilling bits.

The forward and backward movements of the rotary cutter 31 can be inverted by the rotation of the driving source 34 such as a motor provided in the travelling body 35.

By changing the drilling radius of the rotary cutter 31, the expansion drilling exceeding the building frame width of the underground structure 10 can be achieved.

[2.3] Operation Control of Drilling Apparatus

A plurality of the drilling apparatuses 30 can be controlled so as to be able to uniformly drill the just-underground at the bottom part of the underground structure 10.

The uniform drilling means a drilling operation to move the plurality of drilling apparatuses 30 so as to be able to maintain the favorable horizontality of the underground structure 10.

Specifically, when the underground structure 10 has an oblong planar shape, the respective drilling apparatuses 30 are allowed to operate so that the drilling apparatus 30 provided on the long side and the drilling apparatus 30 provided on the short side start the drilling operation at the same timing from the starting point position and reach a turning point at the same timing.

The respective drilling apparatuses 30 can be operated automatically or manually.

(Underground-Structure Construction Method)

Next, the following section will describe a method of constructing the underground structure 10.

[1] Open-Cut Operation and Formation of Reference Horizontal Plane

Description will be made with reference to (a) in FIG. 3. The ground 40 under which the underground structure 10 is planned to be sunk is open-cut to form a mortar-shaped working hole 41.

The working hole 41 is sized so that the lateral width L₁ above the ground is larger than the lateral width L₂ of the bottom part and the lateral width L₂ of the bottom part of the working hole 41 is larger than the lateral width L₃ of the underground structure 10.

The working hole 41 has a depth H that is sufficient to correspond to the height of about one span of the underground structure 10 (about 3 to 6 m).

When the working hole 41 is formed, a horizontal reference area 42 having a continuous closed shape is formed that is obtained by horizontally levelling the edge ground of the bottom part of the working hole 41.

The horizontal reference area 42 functions as a drilling initiation plane when the bottom frame 11 is placed and the underground structure 10 is sunk. Thus, it is very important that the horizontal reference area 42 is sufficiently compacted to have a high accuracy.

[2] Assembly of Bottom Frame

Description will be described with reference to (a) in FIG. 3 and FIG. 4. The hung steel material such as H steel is lowered into the working hole 41. Then, the bottom frame 11 is assembled to have a closed shape on the horizontal reference area 42.

When the bottom frame 11 is placed on the horizontal reference area 42, the bottom frame 11 is assembled while confirming the horizontal levels of the respective steel materials.

The bottom frame 11 can maintain the high horizontality by the continuous horizontal reference area 42 formed to have a high horizontal accuracy in advance over the entire length and the horizontal reference area 42 having a surface contact with the lower flange 13 to support the bottom frame 11 so that the weight of the bottom frame 11 is uniformly distributed.

[3] Assembly of Drilling Apparatus

Description will be made with reference to FIG. 2 and (b) in FIG. 3. In the working hole 41, a plurality of the drilling apparatuses 30 are assembled on the horizontally-installed bottom frame 11 with an interval thereamong.

The respective drilling apparatuses 30 are assembled to the inner side of the bottom frame 11.

[4] Construction of Underground Structure

With reference to (b) in FIG. 3 and FIG. 5, the following section will describe a step of constructing a part corresponding to one span of the underground structure 10 (height of about 5 to 6m). The reinforcing bar 50 is assembled on the upper flange 12 of the bottom frame 11 and a formwork 51 is assembled so as to face the reinforcing bar 50.

After the formwork 51 is assembled over the entire length of the bottom frame 11, the formwork 51 is uniformly filled with concrete 53 sent from a plurality of positions.

The placed concrete 53 increases the weight loaded to the bottom frame 11. However, the continuous lower flange 13 of the bottom frame 11 has a surface contact with the continuous horizontal reference area 42 to uniformly distribute the loaded weight. Therefore, the bottom frame 11 can maintain the high horizontality.

After the concrete 53 is cured, the formwork 51 is disassembled and removed.

Since the working hole 41 is formed to be larger than the outer shape of the underground structure 10, a series of operations can be smoothly and efficiently carried out, including an operation to assemble the reinforcing bar 50, an operation to assemble the formwork 51, and an operation to disassemble the formwork 51.

[5] Backfilling of Annular Space

As shown in (c) in FIG. 3, the space in working hole 41 is divided into two inner and outer spaces to sandwich the underground structure 10, forming the internal space 41 a of the underground structure 10 and the outer annular space 41 b of the underground structure 10.

Backfill soil 43 is placed on the entire area of the annular space 41 b to backfill the interior until the backfilling reaches the earth surface level.

[6] Sinking of Underground Structure by Self-Weight

Description will be made with reference to FIG. 1. The operation to use a plurality of the drilling apparatuses 30 to drill the just-underground of the bottom rail 11 and the operation of drilling and the earth removal of the internal ground 43 of the underground structure 10 are carried out repeatedly, thereby sinking the underground structure 10 by its self-weight.

[6.1] Uniform Drilling by Drilling Apparatus

Each drilling apparatus 30 starts the uniform drilling of the continuous horizontal reference area 42 shown in FIG. 5 while repeating the reciprocation along bottom rail 11. The drilled earth and sand are discharged into the internal space 41 a of the underground structure 10.

The plurality of the drilling apparatuses 30 uniformly drill the just-underground of the bottom part of the underground structure 10. Thus, the underground structure 10 is allowed to sink due to its self-weight while maintaining the favorable horizontality at the start of the sinking.

In the initial stage of the sinking, the earth pressure due to the backfill soil 43 acts on the outer peripheral surface of the underground structure 10. However, the weight of the underground structure 10 overwhelms the earth pressure, thus preventing the backfill soil 43 from hindering the underground structure 10 from sinking.

The backfill soil 43 abutted to the outer peripheral surface of the underground structure 10 is significantly effective to prevent the entire underground structure 10 from being laterally moved or being inclined during the initial stage of the sinking of the underground structure 10.

In other words, the backfill soil 43 significantly contributes to the sinking of the underground structure 10 while maintaining the high horizontality.

In the case of the conventional construction method, the space 44 formed between the expanded and drilled pore wall and the outer wall face of the underground structure 10 was filled with a fine grain aggregate for reducing the frictional resistance.

The drilling apparatus 30 drills the just-underground until the building frame width of the underground structure 10 is exceeded to thereby form the space 44 functioning as an insulating structure between the pore wall subjected to the expansion drilling and the outer wall face of the underground structure 10. Therefore, this construction method does not require the operation to fill the space 44 with the fine grain aggregate.

[6.2] Drilling of Internal Ground and Discharge of Earth and Sand

The internal ground 43 of the underground structure 10 is drilled using a known drilling apparatus.

In order to improve the earth removal of the drilling apparatus 30, the neighborhood of the underground structure 10 is drilled to form an annular groove along the underground structure 10. Specifically, when a drilling operation is performed by the drilling apparatus 30, the annular groove is desirably drilled in advance so that a groove bottom is positioned at a lower position than the just-underground of the bottom part of the underground structure 10.

The drilled earth and sand in the underground structure 10 is discharged onto the ground by a clam shell bucket 44 or a known earth removal means such as a conveyor delivery or a damped motor lorry.

[6.3] Extension of Underground Structure

In parallel with the sinking of the underground structure 10 or after the sinking of the underground structure 10, the building frame is additionally provided and extended to the upper part of the underground structure 10.

As required, a slab 17 is horizontally constructed in the underground structure 10.

The slab 17 also may be formed by providing an opening 17 a at a part of the slab 17 to remove earth onto the ground through this opening 17 a.

The slab 17 is used as the installation floor of the electric power facility 18 to supply power to the drilling apparatus 30.

[6.4] Removal of Drilling Apparatus

After the underground structure 10 is sunk to a predetermined depth, the drilling apparatus 30 is removed from the bottom frame 11. The removed drilling apparatus 30 is reused.

The underground structure 10 is used for various applications with a slab provided on the bottom part.

[7] Regarding Horizontality of Underground Structure

The extended building frame of the underground structure 10 causes an increase of the loaded weight on the bottom rail 11. However, according to the present invention, the underground structure 10 can be sunk while maintaining the horizontality of the underground structure 10 due to the following reasons.

A ratio of a region occupied by the drilling apparatus 30 to the entire length of the bottom rail 11 is very small. Therefore, the underground structure 10's self-weight is distributed and uniformly supported by the just-underground maintaining the horizontality over the entire length of the bottom rail 11.

Therefore, the underground structure 10's self-weight does not directly act on the drilling apparatus 30. The drilling apparatus 30 can efficiently drill the just-underground firmly compacted by the underground structure 10's self-weight.

As described above, according to this construction method, the underground structure 10 can be sunk while maintaining the high horizontality. This consequently can eliminate any of a high-cost subsidence control apparatus and a foundation pile for supporting this and a concrete foundation used in the conventional construction method, thus achieving a shorter construction period and a significantly-reduced construction cost.

(Reference Numerals) 10 Underground structure 11 Bottom frame 20 Subsidence control apparatus 30 Drilling apparatus 31 Rotary cutter 40 Ground 41 Working hole  41a Internal space  41b Annular space 42 Horizontal reference area 

What is claimed is:
 1. An underground-structure construction method of arranging underground an underground structure by: causing a plurality of drilling apparatuses, which are reciprocatably engaged with a bottom frame of closed shape made of concrete of the underground structure, to travel in a reciprocating manner so as to drill a just-underground of a bottom part of the underground structure; discharging, to an earth surface, excavated earth and sand of an internal ground on an internal side of the underground structure; and extending a building frame of an upper part of the underground structure, the underground-structure construction method comprising the steps of: open-cutting a ground so as to form a mortar-shaped working hole larger in size than the underground structure; forming a horizontal reference area of closed shape in an edge ground of a bottom part of the working hole; arranging a bottom frame on the horizontal reference area in the working hole so as to form the bottom frame into a closed shape; building a first stage building frame of the underground structure on the bottom frame in the working hole; and backfilling, with backfill soil, an annular-shaped outer space out of inner and outer spaces as a result of dividing the working hole at the underground structure, wherein the plurality of drilling apparatuses are caused to uniformly drill, from the horizontal reference area as a drilling initiation plane, the just-underground of the bottom part of the underground structure.
 2. The underground-structure construction method according to claim 1 wherein the bottom frame has a continuous lower flange formed thereon, and a self-weight of the underground structure is supported, through an entire lower surface of the lower flange, by the just-underground of the bottom part of the underground structure.
 3. The underground-structure construction method according to claim 1 wherein in parallel with causing the plurality of drilling apparatuses engaged with the bottom frame to uniformly drill the just-underground of the bottom part of the underground structure, the internal ground on the internal side of the underground structure is excavated so that excavated earth of the internal ground is discharged to the earth surface.
 4. The underground-structure construction method according to claim 1 wherein the plurality of drilling apparatuses engaged with the bottom frame are caused to perform a width-expansion drilling beyond a width of the building frame of the underground structure, so that the underground structure is allowed to sink while a space is formed for insulation between a hole wall as a result of the width-expansion drilling and an outer wall surface of the underground structure.
 5. The underground-structure construction method according to claim 2 wherein the plurality of drilling apparatuses engaged with the bottom frame are caused to perform a width-expansion drilling beyond a width of the building frame of the underground structure, so that the underground structure is allowed to sink while a space is formed for insulation between a hole wall as a result of the width-expansion drilling and an outer wall surface of the underground structure.
 6. The underground-structure construction method according to claim 3 wherein the plurality of drilling apparatuses engaged with the bottom frame are caused to perform a width-expansion drilling beyond a width of the building frame of the underground structure, so that the underground structure is allowed to sink while a space is formed for insulation between a hole wall as a result of the width-expansion drilling and an outer wall surface of the underground structure.
 7. The underground-structure construction method according to claim 1 wherein each of the plurality of drilling apparatuses includes: a self-propellable travelling body engaged with the bottom frame; and a rotary cutter arranged in the travelling body rotatably around a vertical axis, so that each of the plurality of drilling apparatuses drills the just-underground of the bottom part of the underground structure through the rotary cutter's rotation and travel along the bottom frame.
 8. The underground-structure construction method according to claim 2 wherein each of the plurality of drilling apparatuses includes: a self-propellable travelling body engaged with the bottom frame; and a rotary cutter arranged in the travelling body rotatably around a vertical axis, so that each of the plurality of drilling apparatuses drills the just-underground of the bottom part of the underground structure through the rotary cutter's rotation and travel along the bottom frame.
 9. The underground-structure construction method according to claim 3 wherein each of the plurality of drilling apparatuses includes: a self-propellable travelling body engaged with the bottom frame; and a rotary cutter arranged in the travelling body rotatably around a vertical axis, so that each of the plurality of drilling apparatuses drills the just-underground of the bottom part of the underground structure through the rotary cutter's rotation and travel along the bottom frame.
 10. The underground-structure construction method according to claim 4 wherein each of the plurality of drilling apparatuses includes: a self-propellable travelling body engaged with the bottom frame; and a rotary cutter arranged in the travelling body rotatably around a vertical axis, so that each of the plurality of drilling apparatuses drills the just-underground of the bottom part of the underground structure through the rotary cutter's rotation and travel along the bottom frame.
 11. The underground-structure construction method according to claim 5 wherein each of the plurality of drilling apparatuses includes: a self-propellable travelling body engaged with the bottom frame; and a rotary cutter arranged in the travelling body rotatably around a vertical axis, so that each of the plurality of drilling apparatuses drills the just-underground of the bottom part of the underground structure through the rotary cutter's rotation and travel along the bottom frame.
 12. The underground-structure construction method according to claim 6 wherein each of the plurality of drilling apparatuses includes: a self-propellable travelling body engaged with the bottom frame; and a rotary cutter arranged in the travelling body rotatably around a vertical axis, so that each of the plurality of drilling apparatuses drills the just-underground of the bottom part of the underground structure through the rotary cutter's rotation and travel along the bottom frame. 